Android: Add @Keep annotation to BooleanSupplier

I should at least follow the conventions I set myself... (See a8d385c)

Source/Android/app/src/main/java/org/dolphinemu/dolphinemu/utils/BooleanSupplier.java

@@ -1,6 +1,9 @@
 package org.dolphinemu.dolphinemu.utils;
 
+import androidx.annotation.Keep;
+
 public interface BooleanSupplier
 {
+  @Keep
   boolean get();
 }